Ring networks provide a continuous transmission structure between nodes connected in tandem. Each node receives transmission from an adjacent upstream node, extracts information destined for it, re-transmits information destined for other nodes located further downstream, and adds information destined for other nodes on the ring.
Two distinct categories of ring networks as described in "Definitions of Ring Architectures", Contribution T1X1.5/90-179, to T1 Standards Project-T1X1.5 Nov. 5, 1988, by J. Sosnosky et al., are commonly referred to as unidirectional and bidirectional. Unidirectional networks, as discussed in my prior patent application Ser. No. 07/577,219, are characterized by having upstream and downstream information transported between nodes over opposite transmission routes yet on the same transmission link. Conversely, bidirectional ring networks, upon which may present invention is based, are characterized as having upstream and downstream information transported between nodes over the same transmission route, but on two separate transmission links.
A drawback of both categories of ring networks is that a break in the ring would prevent any node upstream of the break from communicating with any node downstream of the break over the path containing the break. Similarly, a complete failure of a node would have the same effect as a break in the ring physical link. To improve the survivability of ring networks under such abnormal conditions, self-healing features have been implemented which provide resiliency to network failures.
Unidirectional and bidirectional networks can also differ in the self-healing technique and the number of protection links required to provide protection capability in the event of a break in the transmission link or a node failure. Unidirectional rings generally employ a signal duplication technique and one additional transmission link for protection, but bidirectional rings generally employ a loop-back technique and two additional protection transmission links which remain unused under normal operation.
Although unidirectional and bidirectional networks are distinct structures generally employing a different number of transmission links and a different self-healing technique, communication between nodes in both networks is accomplished in a similar fashion. For both ring structures, the same signal structure consisting of a predetermined number of subrate multiplexed channels operating at a fixed rate is transported around the ring between all nodes. Since all links and nodes in both unidirectional and bidirectional networks transport signals having the same fixed rate and multiplexed subchannel structure, all links and nodes are required to have adequate transmission capacity to accommodate the total rate of all multiplexed subchannels. This high bandwidth requirement imposed upon network nodes and links makes network deployment cost-prohibitive. Given the high bandwidth of the total multiplexed signal structure, there is little spare bandwidth to meet upgrade requests.
These shortcomings as well as other limitations and deficiencies of unidirectional ring networks have been obviated by the invention of my prior patent application Ser. No. 07/577,219. As disclosed in my prior application, a unidirectional ring network is configured as logical rings each composed of segments interconnected at the subchannel level by crossconnect nodes such that each segment can operate at a different rate, can have a different signal structure, and can contain a different number of subchannels within the signal transported on each segment.
The object of my present invention is to overcome the deficiencies of bidirectional self-healing ring networks by configuring these networks as segments interconnected at the subchannel level by crossconnect nodes to form logical-ring networks.